1. Field of the Invention
Apparatuses and devices consistent with the present invention relate to an electric discharge lamp and, more particularly, to an electric discharge lamp including a ceramic luminous tube.
2. Description of the Related Art
For example, in a related art headlight for a vehicle, an incandescent lamp (incandescent bulb), a halogen lamp (halogen bulb) or an electric discharge lamp (electric discharge bulb) is used for the luminous source.
In a headlight using an incandescent lamp or a halogen lamp for the luminous source, a filament of the incandescent lamp or the halogen lamp forms a rod-shaped luminous portion and is substantially uniformly luminous. Accordingly, when the incandescent lamp or the halogen lamp is used in a reflection type lighting device, which includes a reflector, a light distribution control can be executed by changing a shape of a reflecting face of the reflector.
On the other hand, in a related art headlight having an electric discharge lamp, since a quantity of light of the electric discharge lamp is larger than that of the incandescent lamp or the halogen lamp, the luminance can be enhanced. Further, the life of the electric discharge lamp is longer than that of the incandescent lamp or the halogen lamp.
As described above, the luminance of the electric discharge lamp is higher than that of the incandescent lamp or the halogen lamp and the life of the electric discharge lamp is longer than that of the incandescent lamp or the halogen lamp. Thus, headlights including the electric discharge lamp have come into wide use recently.
Concerning the related art electric discharge lamp, a luminous tube made of glass is used. A pair of electrodes are held in the luminous tube and a rare gas is filled in the luminous tube. However, metallic halide filled in the luminous tube tends to corrode the luminous tube made of glass. Accordingly, blackening or devitrification is caused, and it becomes difficult to obtain a proper light distribution. Further, as the corrosion increases, the life of the electric discharge lamp is shortened.
Thus, a related art electric discharge lamp has been proposed which has a luminous tube made of ceramics instead of the glass luminous tube. For example, Japanese Patent Unexamined Publication JP-A-2004-103461 describes a related art electric discharge lamp including a ceramic luminous tube.
In the related electric discharge lamp described in the JP-A-2004-103461, a pair of electrodes are held by the ceramic luminous tube and a hermetic space is formed in the ceramic luminous tube. The hermetic space formed in the ceramic luminous tube is filled with rare gas such as xenon gas and metallic halide. The ceramic luminous tube is covered with an outer tube made of glass and the hermetic space is also formed between the ceramic luminous tube and the outer tube.
Since the ceramic luminous tube is stable with respect to metallic halide, the life of the electric discharge lamp described above is advantageously longer than that of the electric discharge lamp having a glass luminous tube.
However, there are also a number of disadvantages. For example, in the electric discharge lamp, at a time of electrical discharge, a so-called arc-bend is generated in which a central portion of an arc generated between a pair of electrodes is bent upward. Accordingly, a temperature of an upper portion of the luminous tube becomes higher than a temperature of other portions.
When the temperature of the upper portion of the luminous tube becomes higher than the temperatures of the other portions, there is a possibility of generating cracks in the luminous tube by thermal stress depending upon the heat resistance and the thermal shock property of the luminous tube.
Since it is known that by lowering the pressure of the rare gas, such as xenon gas filled in the luminous tube, the arc-bend is suppressed, it has been considered to decrease the pressure of the rare gas in order to suppress the generation of arc-bend. However, when the pressure of rare gas is decreased, a disadvantage results in that the luminous efficiency is deteriorated.